FIG. 23 shows a conventional capacitance sensing network 2300 that includes transmit (TX) electrodes 2302-0 to -2 and receive (RX) electrodes 2304-0 to -2 formed below the TX electrodes (2302-0 to -2). In operation, a TX electrode (2302-0 to -2) can be driven by a transmit signal. A mutual conductance between a driven TX electrode and RX electrodes (2304-0 to -2) can be sensed. The presence of an object (e.g., a finger) can result in a change in such a mutual capacitance, which can be sensed as an input event. In some conventional systems, the self-capacitance to ground of some or all electrodes can be sensed.
FIG. 24 shows another conventional capacitance sensing network 2400 that includes TX electrodes 2402-0/1 and RX electrodes 2404-0/1. TX electrodes 2402-0/1 and RX electrodes 2404-0/1 are formed by diamond shaped plates (one shown as 2406). RX electrodes 2404-0/1 are formed by plates being conductively connected to one another in one direction (shown as Y). TX electrodes 2402-0/1 are formed by plates being conductively connected to one another in a perpendicular direction (shown as X) by conductive jumpers (or overpasses) (one shown as 2408). Sensing operations for sensing network 2400 can occur in the manner described for FIG. 23.
FIG. 25 is a schematic diagram showing a sensing model of a conventional device containing a sensing network like that shown in FIG. 23 or 24. FIG. 25 shows capacitance elements that can exist between a TX electrode 2502 and a RX electrode 2504 in a sense operation. Capacitance Cm (Intrinsic) can be a mutual capacitance between TX and RX in the absence of an object, giving rise to a baseline current I_baseline. Capacitance Cm (object, neg value) can be a capacitance change (in this case a negative change) resulting from the proximity of a human body (e.g., finger), giving rise to a sense current component (+I_signal). Capacitance Cs (object) can be a “self” capacitance between a sensed object (e.g., finger) and a device “ground”. It is understood that a device ground may not be a true ground potential, and can arise from a device being placed in a non-conductive environment. A capacitance Cf can be a capacitance that exists between the device and a grounded human body when the device ground is decoupled from the human body ground.
As shown, the absence of a significant Cf can give rise to a counter sense current component (−I_signal), which can work against the strength of sense current +I_signal. A sufficiently large counter sense current (−I_signal) can give rise to a non-response (i.e., failure to sense object) or false touch event.